The direct current (DC) offset is a typical defect of radio frequency (RF) reception subsystems, more particularly, direct conversion reception sub systems, also called zero intermediate frequency (ZIF). It is in typical to compensate for DC offset before performing a demodulation operation in order to improve overall reception processing performance. An excessive DC offset that is not compensated for in the analog part of the reception subsystem may use a large analog-to-digital converter dynamic range, which may not be desirable.
One cause of this DC offset originates in leakage from the local oscillator signal (transposition signal), which is intended for the transposition stage of the subsystem (mixer) at the input of the low noise amplifier of this subsystem and at the input of the mixer itself. This leakage signal is consequently amplified and multiplied by itself in the mixer to create this DC offset. Moreover, all the elements of the subsystem that have a gain naturally amplify this DC offset.
Essentially, two types of approaches have been disclosed to address this DC offset. A first approach includes using a high-pass filter operation. However, such an approach uses precise control of the cut-off frequency and of the phase response of the filter, which can lead to relatively costly and bulky filter designs. Moreover, analog filters are used for the calibration of the cut-off frequencies. In order to avoid a factory calibration, which is costly, it is may be helpful to use an on-chip calibration. However, such tunable analog filters use a surface area that is far greater than that of the chip itself.
The second type of approach is aimed more at compensating for the DC offset, in an analog or digital manner, rather than eliminating it through a filtering operation. One way to estimate the DC offset is to do it during a silence period, i.e. during a period where no signal is present at the antenna. In fact, in these conditions, the only signal present in the reception subsystem is the DC offset, thereby making it favorable to estimate the DC offset, for example, by using a low-pass filter of low complexity. However, such an approach uses the presence and the knowledge of the silence periods, which implies partial or total synchronization of the network or a network with low traffic.
However, in certain cases, the silence periods do not exist or are very short by comparison to the actual transmission periods, as is the case in, for example, the cellular systems operating in accordance with the Worldwide Interoperability for Microwave Access (WiMAX) standards. WiMAX is a family of standards that define high bit rate connections by microwave channel, being intended mainly for point-multipoint architectures. The WiMAX standards include in particular the standards of the IEEE 802.16 family.
Another method of estimating the DC offset is to first synchronize the receiver and then perform an estimation of this DC offset by using, for example, a Forward Fourier Transform (FFT) module of the receiver, for example, an Orthogonal Frequency Division Multiplexing (OFDM) receiver. However, with such an approach, the synchronization is performed in the presence of a high DC offset level, which can sometimes range up to 50% of the operating range of the analog-to-digital converter. This implies the use of digital filters to eliminate the DC offset while performing a cell or incoming signal search and while reducing the gain of the analog stage in order to avoid a saturation of the analog-to-digital conversion stage. Consequently, the level of the signal and the level of the signal-to-noise ratio (SNR) are reduced, thereby leading to a reduction in the sensitivity of the receiver.
Most of the wireless communication standards using a modulation of the OFDM type have taken account of this problem of self-mixing (self-transposition) of the local oscillator signal, giving rise to the creation of the DC offset. In practice, in such transmission standards, there is no signal transmitted on the zero frequency (DC frequency). Consequently, the DC offset has no effect provided that the carrier frequency of the transmitter and the carrier frequency of the receiver are perfectly synchronized. However, on the one hand, this is not the case before synchronization and, on the other hand, a perfect synchronization is consequently used, which is an ideal case, i.e. not arising in practice.
Also, the DC offset level is typically reduced in the OFDM systems to address the impact of the carrier frequency offset and of inter-carrier interference. Furthermore, the presence of this DC offset component in the digital signal has an impact on the dimensioning of the digital part of the receiver. Consequently, most of the RF receivers, and in particular the receivers of the direct conversion type, are very carefully designed so as to minimize this self-transposition of the local oscillator signal and reduce the DC offset level.
However, such a minimization is not yet sufficient and consequently uses an estimation of the DC offset level in order to compensate for it as much as possible. Moreover, considering the context of a WiMAX receiver, certain applications use the processing of the DC offset to be performed before any synchronization, the synchronization being possible only if the imperfections of the signal are reduced.